The present invention relates generally to wireless communication systems and in particular to a system for distributing wireless system signals between a central base station and remotely deployed transceiving equipment.
Wireless communication networks, such as cellular mobile telephone and Personal Communications Services (PCS), continue to enjoy wide spread growth and popularity. There is often times a need in such systems to provide increasingly greater call handling capacity, as well as to accommodate higher peak usage. Emerging Personal Communication Services (PCS) networks, presently still in the stages of being implemented, demand additional design considerations such as low build out cost as they must compete with entrenched cellular networks.
Several approaches have been adopted for deploying such networks. One approach is to increase the coverage area afforded by a given system by increasing the antenna tower height and transmit power level beyond conventionally accepted norms. However, such solutions often increase the number of xe2x80x9cblindxe2x80x9d spots in areas that include a number of tall buildings, hills, or other natural obstructions to radio propagation.
Alternatively, a relatively large number of base stations may be deployed with smaller radio coverage xe2x80x9cfootprintsxe2x80x9d. While this avoids blind spots, it greatly increases the total capital cost for base station transceiving equipment which may be $200,000 or more per cell site.
Rather than deploy base station equipment in each relatively small cell (which would be relatively cost prohibitive), broadband distribution cable and/or fiber optic networks can be used to connect the antennas to centrally located base station equipment. For example, a suggestion has been made in U.S. Pat. No. 5,381,459 to use cable television networks to distribute wireless communication signals between base transceiver system (BTS) equipment and remote transceiver sites located at each cell. This approach couples the transceiver signals over an existing coaxial cable television network using time or frequency division multiplexing in order to avoid interference with the other signals being carried by the cable, such as cable television (CATV) signals.
Recently, other types of broadband distribution networks have also been proposed. Such networks consist of optical fiber transmission media which can directly distribute signals between centrally located base transceiver system (BTS) equipment and remotely located transceiver equipment. See, for example, our co-pending U.S. patent application Ser. No. 09/256,244 entitled xe2x80x9cOptical Simulcast Network with Centralized Call Processing,xe2x80x9d filed Feb. 23, 1999.
There is also presently a demand by the customers of such cellular telephone systems for digital modulation techniques, such as Code Division Multiple Access (CDMA). In these CDMA systems, such as the IS-95B system being used widely in the United States a common frequency band is used to support communication between multiple mobile subscriber units and base stations. With this technique, signals occupying a common carrier frequency are discriminated at a receiving terminal (which may either be the base station or is the mobile unit) based on the use of pseudo random noise (PN) codes. In particular, transmitting terminals use different PN codes or PN code phase offsets to produce signals that may be separately received. The mobile unit is then provided with a list of carrier signal codes and phase offsets corresponding to neighboring base stations surrounding the base station through which communication is established. The mobile unit is also equipped with a searching function that allows it to track the strength of the carrier signals generated from a group of the neighboring base stations.
The carrier signal power levels used with most cellular and PCS systems are selected to work with mobile units that travel among cells that provide coverage over an area of several miles, such as is typically encountered along a suburban highway. However, certain other challenges are presented when attempting to provide coverage in a densely populated city and/or in other environments where coverage is to be limited to a building. Within such environments, signal reflections off of building infrastructure are quite common, and typical signal fading studies prove that line-of-sight (LOS) propagation is not typically the dominant propagation mode. This is in distinct contrast to the typical situation in a suburban environment where a line-of-sight does typically exist between the mobile subscriber unit and the neighboring base stations. Within the building, metal, concrete, and other structures typically provide a signal fading characteristic for over-the-air propagation which in turn requires the wireless signal transmissions to be carried out at power levels which must be higher than would otherwise be necessary. This in turn has negative effect on the ability of the wireless system as a whole to provide service to a maximum number of users. This situation is particularly acute in systems that make use of CDMA-type signalling in which the overall system capacity is a function of the interference generated by individual signal power outputs required for individual subscribers.
The present invention is a solution for providing an improved fading margin environment for in-building wireless coverage. An optical fiber or other available broadband signal distribution network is used to distribute signals between centrally located base transceiver station (BTS) equipment and remotely located transceiver equipment referred to herein as a cable microcell integrator (CMI). The CMIs are deployed in a configuration to provide radio frequency coverage in a desired microcell area such as a building floor. In a preferred embodiment, a leaky coaxial cable is used as a radiating element within the microcell.
In a preferred embodiment, a radio frequency signal splitter or power divider is used with each CMI to divide transmit power evenly to a pair of leaky coaxial radiating elements run along each of the sides of a building floor. A duplexer is then used to combine the receive link signals with the transmit link signals. The combined signal on the duplexer port is then in turn coupled to the radiating coaxial cables.
In a preferred embodiment, the leaky coaxial cable may be strung above ceiling tiles, out of sight, or may be mounted as radio frequency propagation strips along the metal guides for suspended ceilings.
The radiating leaky coaxial cable selected is typically one-half inch plenum rated 50 ohm coaxial cable with radiating apertures located along its link. The leaky coaxial cable can be terminated with a five watt load or further antenna element.
There are several advantages that result from using the leaky coaxial cable with relatively high power transmit levels inside a building.
Complete coverage of the building floor is provided while changing the fading characteristics of the RF signals. In particular, the fading characteristics of the building now more closely approximate line-of-sight type coverage areas as opposed to the multi-path environment typically encountered in a building.
A second advantage results from the use of a primary receive port for one side of a building floor and a diversity receive port for another side of the building floor. The lack of a second diversity reception port within the building has been found not to be a detriment to system performance since the multi-path environment, along with the reception properties of the leaky coaxial cable itself creates a multiple delayed signal versions that are received back a rake receiver at the base station. This allows for optimal signal ratio combining with the various fingers of the receiver operating similarly to diversity receivers. Therefore, by breaking a building into two distinct microcell regions, interference generated by the mobile subscriber units can be further reduced.
Also, in the preferred embodiment, a single CMI can provide coverage to multiple floors or multiple sections of the same floor. In such cases, signal power may be split at the CMI to explicitly feed radio frequency energy to each floor section. In a case where two different floors of a building are fed from the same CMI, floor penetration loss of the radio signals is not incurred. Rather, floor penetration is instead used to help provide separation between the microcells thereby created, mitigating interference between such sectors.
In this manner, radio coverage is encouraged to remain line-of-sight so that fading characteristics remain benign. The two receive channels can therefore be used to greater advantage by allowing them to cover different parts of the building. This also provides an advantage to the signal powers on the reverse link as the power is also split in that signal propagating direction as well.